1. Field of the Invention
The present invention generally relates to oversampled analog-to-digital (A/D) converters and, more particularly, to wide dynamic range delta sigma A/D converters with precise gain tracking. The gain tracking may be accomplished with either analog or digital techniques. A/D converters incorporating the features of the invention have particular application in multiple channel systems wherein an A/D converter is employed in each channel and each A/D converter must precisely follow the same compression and/or time-gain variation curve.
2. General Description of the Prior Art
With recent advances in the use of digital signalprocessing techniques for analog signals, many refinements in the basic and well-known process of analog-to-digital (A/D) conversion have been required. A/D converters have become very sophisticated, providing conversions at high speeds and with increasing accuracy. High resolution A/D signal conversion can be achieved with lower resolution components through the use of oversampled interpolative (or delta sigma) modulation followed by digital low pass filtering and decimation. Oversampling refers to operation of the modulator at a rate many times greater than the Nyquist rate, whereas decimation refers to reduction of the clock rate by periodic deletion of samples.
Delta sigma modulators (sometimes referred to as sigma delta modulators) have been used in A/D converters for some time. Detailed general information can be obtained from the following technical articles which are hereby incorporated by reference:
1) "A Use of Limit Cycle Oscillators to Obtain Robust Analog to Digital Converters", J. C. Candy, IEEE Transactions on Communications, vol. COM-22, no. 3, March 1974, pp. 298-305. PA0 2) "Using Triangularly Weighted Interpolation to Get 13-Bit PCM from a Sigma-Delta Modulator", J. C. Candy et al., IEEE Transactions on Communications, vol. COM-24, no. 11, November 1976, pp. 1268-1275. PA0 3) "A Use of Double Integration in Sigma Delta Modulation", J. C. Candy, IEEE Transactions on CommunicatIons, vol. COM-33, no. 3, March 1985, pp. 249-258.
Broadly described, a delta sigma A/D converter uses an internal A/D converter of modest resolution and a complementary digital-to-analog (D/A) converter in a feedback loop. The feedback loop increases accuracy of the A/D converter in a manner consistent with the high speed operation afforded by the internal A/D converter. In theory, any error in linearity or resolution caused by the D/A converter is effectively added to the input signal and appears at the output without attenuation.
Substantial effort has been expended in the field of oversampled A/D converter design to develop a delta sigma A/D converter with an increased dynamic range. Delta sigma A/D converters employ an analog integrator at the input to assure that the average of the digital output bits equals the average of the signal. This makes them inherently linear (assuming that the integrator is linear); however, a linear A/D converter wastes resolution in cases where the noise level is not uniform with signal amplitude. Attempts to match delta sigma A/D converters to applications that have non-uniform noise levels have used companding, but use of companding may make it necessary to restore linearity. This requires an exactly invertible companding characteristic.
Active imaging systems such as radar, sonar and ultrasound systems require very large dynamic range since reflections from nearby objects are generally of much larger amplitude than reflections from objects farther away. At present, this problem is solved by introducing some form of compression before the analog-to-digital (A/D) converter or by using an A/D converter with a relatively large number of bits of resolution. Another solution that is often used, particularly in ultrasound systems, is to introduce a time dependent amplification so as to equalize the amplitudes of the reflections. All of these approaches have drawbacks. In the case of an array of receivers, it is difficult to make all channels follow an identical compression curve. High resolution A/D converters are expensive and require much power, and again in the case of an array of receivers, it is difficult to make all the channel gains track each other as the gains are changing.
Nonlinear gain compression curves are usually derived from nonlinear analog components (such as diodes) which are subject to component variations. Similarly, electronically variable gain elements depend on analog components which are subject to variations. These variations make it difficult to form high quality beams from phased arrays.